Acoustimeter



ACOUSTIMETER Filed Dec. 10, 1926 INVENTOR Ralph FA/a rr/s 2W, )uwvcwmdATTORNEYS Patented May 14, 1929.

UN'ETED STATES Parent orrica.

RALPH FORB'USH NORRIS, F MADISON, WISCONSIN, ASSIGNOR '10 0. BURGESSLABORATORIES, INC., OF MADISON, WISCONSIN, A CORPORATION OF DELAWARE.

ACOUSTIME'IER.

Application filed December 10, 1926. Serial No. 154,018.

This invention relates to methods and apparatus for measuring sounds andmore particularly to a method of determining the r sound-absorbing valueof various materials. An object of my invention is the provision ofmeans for emitting sounds of known acoustical power in combination witha currentmeasuring device by means of which the eurrent employed to emitthe sound may be measured, and the sound prevailing at a fixed point mayalso be translated into electric current and measured. A further objectat my invention is to provide a simple, accurate and eflicient apparatusfor scientifically studying sound transmission, acoustics of rooms,sound absorbing qualities of materials, and especially those materialsused for interior walls, and further to provide a simple means formeasuring the intensity of sound; It is a still further object of myinvention to provide means for quickly calibrating the acoustimeter sothat l accurate readings'may be quickly made.

- In practising the process I employ novel apparatus to produce soundsof known acous tic powers by means of an electrically operat ed tuningfork and a loud speaker, and I provide calibratable means for measuringtheir effect in rooms. The average absorb- 3o ing Value of the surfacesof a room may thus be readily determined from these measurements. Havingdetermined the average absorbing value of a room its acoustics maybechanged to produce desired effects bythe 5 addition or subtraction ofabsorbing material. The sound resistance of a wall or other structuremay also be measured by placing the sound producer on one side of thewall and the sound meter onthe opposite side. 40 The reading obtained onthe meter is propor' tional to the intensity of sound transmitted by thewall. 7

-At the present time two methods are used for measuring thereverberation of rooms. Inthe first method all exposed areas and thevolume ofthe room are carefully measured. The absorption coefiicients ofthe'surfaces are determined by reference to data prepared by ProfessorW. C. Sabine of Harvard University. The areas of the various surfacesare then multiplied by their respective sound absorption coefiicients.The results obtained together with the volume of the room may besubstituted in the formula:

where V=volume, a=absorption, and T time of reverberation. This methodis tedious and inaccurate due to the practical impossibility ofaccurately determining the actual areas of the various materials exposedand the impossibility of obtaining coefficients for every type ofsurface.

In the second method a calibrated organ, pipe is placed in the room tobe tested and sound emitted therefrom and abruptly stopped. The timenecessary for the sound to die out is determined by means of the ear anda stop-watch. While this method gives the period of sound decaydirectly, it is open to the objection that the ear of the observer mustbe calibrated and must always be assumed to be the same.

My novel apparatus eliminates the undesirable features of these methods.In addition to being accurate, it-is easily transportable. In short, itcombines the facility of the second method with extreme simplicity,portability and freedom from Variable human elements, and it also allowsof the. direct measurement of acoustic transmission coefficients of anygiven materials. My novel apparatus consists of three units, a soundproducer, a sound meter,'and a battery box conveniently arranged tooperate the sound meter.

The accompanying drawing representspole double-throw switch 9, asingle-pole double-throw switch 10, a loud speaker 11 and means (notshown) for rotating or other- Wise moving the loudspeaker. A calibrativecircuit 12 comprising a potential-dividing device is connected to oneset of contacts of the' switch 9, and the loud speaker to the other setof contacts.

The sound meter B comprises a sound-frequency amplifier including aninput transformer 13, which controls the grid of a trielectrodethermionic valve or vacuum tube 14,.which is coupled by a resistancethrough a capacity to vacuum tube 16 having a variable grid leakresistance 15, by which the in put to tube 16 ma; be controlled. thuscontrolling the sensitivity of the whole amplifier in a manner wellknown to those skilled in the art. The three vacuum tubes 16, 17 and 18,are coupled by resistances through capacities according to standardpractice. More tubes may be added if desired. The output of tube 18 iscoupled through a stepdown transformer 19, to an audibility meter 20,having anumber of ratio points. The output of the audibility meter isfed as alternating current to thermal-couple 21 which it heats, theheating of the couple causing it to generate a direct currentproportional to the incoming alternating current. 'This di-- rectcurrent is measured by a suitable ineter 22.

The battery box C comprises 90 volts of B battery 23, and 4 volts of Abattery '24:, with appropriate voltmeters 25 and 26,

and rheostats 27 and 28 to properlycontrol the currents to the soundmeter B. r

Before being used to measure sound-absorbing effects, the apparatus mustbe calibrated as follows: First, with the sound-absorbing power of thesurface of the room known, the oscillator is connected'to the soundproducer, and a sound emitted. This sound is picked up by themagnetophone 31, amplified in the sound meter B and a reading securedupon meter 22. The milliammeter 8 is adjusted to a current value whichproduces energy at the input of sound meter B, which is satisfactory forpurposes of operation. Amplification of the sound meter B is adjusted sothat a reading somewhere near mid-scale is obtained on meter 22. Theswitch 9 is then changed from the sound producer 11 to the potentiometer12 and pick-up device 31 disconnected at terminals 30. The movablecontact of the potentiometer is then adjusted so that without changingthe adjustment of the amplifier in sound meter B the same reading issecured upon meter 22 as was previously obtained. The instrument is thencalibrated for any number of subsequent tests. T he current supplied tothe input transformer 13 of the amplifier is thus adjusted to be thesame when it is supplied through potentiometer 12 as when suppliedacoustically from sound producer 11 to pick-up device 31.

The method of operation'to measure the sound absorption of a room forexample, is as follows: The oscillator 7 of the sound prod'ucer A isactuated by means ofbattery 4 through rheostat 5, and then switch 9 isthrown to the calibrating circuit 12. Rheostat 5 is operated until apredetermined current is registered by meter 8. It is presumed that thiscurrent has already been determined, as above described, by calibratingthe instrument in a room of which the sound-absorbing power is known.Terminals 29 of the calibration circuit 12 are then connected toterminals 80 of the sound meter B. The terminals of the battery box Care connected to like lettered terminals of the sound meter B. Thefilament and plate currents of ,vacuum tubes 14, 16, 17 and 18, areadjusted by rheostats 27 and 28 to their proper values as indicated bymeters 25 and 26. The audibility meter 20 is set on a certain pointwhich has been determined to be the correct one for calibrating theinstrument; that is, meter 22' should read approximately at the centerof the scale. Then variable resistance 15 is manipulated until the exactpredetermined reading, pref erably half scale, is shown by meter 22.

Since a current of known value is flowing in calibration circuit 12 ofwhich a definite pro- ;portional part is impressed through connections29 and 30 on the input of the sound meter, and the sound meter isadjusted so that meter 22 registers a definite current output, the soundmeter is now standardized in sensitivity and may be disconnected fromthe calibration circuit at terminals 30.

An acousti-electrical device 31 such as a magnetophone or earphone, isconnected to terminals 30 of the sound meter B as a sound pick-up.Switch 9 of the sound producer A is thrown to the speaker 11 which thenemits a sound of a constant acoustical power and pitch. The speaker isstarted moving or rotating. This speaker should be moving in a way thatcrests or troughs of standing waves will be avoided, as will be furtherexplained hereinafter. I have found that good results may be obtainedwith a speaker vemitting sounds in a direction at an angle, preferablyperpendicular, to an axis about which it is rotating. Two or morespeakers may be used simultaneously and these may rotate about axesmounted at different angles. The movement of the speaker is an importantelement of my invention. This sound is thrown into the room undermeasurement, and a component of the sound together with its reflectionsare received by the magnetophone 31 and are recorded through thecalibrated sound meter by meter 22. Since the sound-producing loudspeaker is rotating on an axis perpendicular to the direction of soundprojection, the sound there may be fluctuations in sound absorption.

The reading recorded by the sound meter is a measure of the intensity towhich a sustained sound of the acoustical power of that generated by thesound producer will rise in that particular room and is inverselyproportional to the total absorption of the room. The relation isexpressed by the equation:

where E is the reading of the sound meter, W is the acoustical power ofthe sound producer, V is the velocity of sound in feet per second at theroom temperature, S isthe total absorbing power of the room and itsfurnishings. W may be determined from the above equation by taking asound meter reading, then adding sound-absorption material of knownvalue to the room and taking a new reading. Simple calculations willgive the acoustical power of the sound producer. To find the period ofreverberation in seconds, thevalue of S is found as above and issubstituted, together with the volume of the room in cubic feet, in theformula:

The sound-absorption value of any material may be measured byintroducing it into a room of known absorbing value and makingmeasurements in a manner similar to those previously described.

In measuring the sound transmitted through partitions the soundproduceris operated on one side of the partition, and the sound meterand pick-up are placed on the opposite side. The readings are directlyproportional to the intensityof the sound transmitted. If in these testsit is desirable to use a sound of different intensity the switch 10 onthe oscillator may be thrown to a different point. Of course, anysuitable source of sound other than that herein described may beemployed.

The sound meter B may be used to determine the absorbing qualities of agiven material by comparing the results obtained on meter 22 with theresults obtained when an absorbing material of known value is used.

Throughout the specification and claims the word room is used in itsgeneric sense to indicate any inclosure.

I claim:

1. The method of measuring sound absorption which comprises measuringthe intensity to which a sustained sound of known acoustical powerrises.

2. The method of measuring the sound absorption of a room whichcomprises measuring the intensity to which a sustained sound of knownacoustical power rises when emitted in said room.

3. The method of measuring the sound absorption of any material whichcomprises measuring the intensity to which a sustained sound of knownacoustical power rises when emitted in a room, introducing said materialinto said room and again measuring the level to which the sustainedsound of known acoustical power rises.

4. The method of avoiding crests or troughs of standing waves whenmaking sound absorption measurements, which comprises emitting sound inmore than one direction, the source of said sound moving in variouspaths.

5. The method of avoiding crest-s or troughs of standing waves, whenmaking sound absorption measurements, which com-' prises continuouslyrotating the direction of sound emission.

6. The method of measuring sound absorption which comprises measuringthe average intensity to which a sustained sound of known acousticalpower rises, and continuously moving, in fixedpaths, the direction ofemission of said sound.

7. In an acoustimeter, a sound-producing speaker rotating about an axis,said speaker having a sound-emitting surface so mounted that the soundwaves will be emitted at an angle to said axis.

8. In an acoustimeter, a sound-producing speaker rotating about an axis,said speaker having a sound-emitting surface so mounted that the soundwaves will be emitted efi'ectively perpendicular to said axis.

9. In an acoustimeter, the combination of an oscillator, a currentmeasuring device, a calibrating circuit, a speaker, means for connectingsaid calibrating circuit tosaid oscil lator, and means for disconnectingsaid calibrating circuit from said oscillator and connecting saidspeaker thereto.

10. In an acoustimeter, the combination of an oscillator, from whichcurrent of a definite value may be obtained, an acoustic-electricaldevice adapted to be connected to said oscillator to measure the currentoutput of said oscillator, and a speaker adapted to be connected to saidoscillator to receive said current of definite value, saidacousti-electroughs of standing waves when making sound-absorptionmeasurements, which comsound-absorption measurements, which comprisesrotating a sound-producing speaker prises moving a sound-producingspeaker in about an axis with said speaker simultane- 11 fixed paths toemit sound in more than one ously emitting a preponderance of sound at 5direction. an angle to said axis.

13. The method of avoiding crests or In testimony whereof I aifix mysignature. troughs of standing waves when making RALPH FORBUSH NORRIS.

